Heat treatment of bearing steel to eliminate retained austenite



United States Patent 3 131,097 HEAT TREATMENT OF BEARING STEEL T0 ELIMINATE RETAINED AUSTENITE Edward R. Mantel, Warren, Mich., assignor to General Motors Corporation, Detroit, Mich, a corporation of Delaware No Drawing. Filed Feb. 23, 1962, Ser. No. 175,300 3 Claims. (Cl. 148-425) This invention relates to steels and more particularly to a method of heat treating bearing steel whereby the retained austenite is completely eliminated for maximum dimensional stability.

Bearing steels for use at higher than ambient tempera tures or in applications where the retained austenite must be eliminated are desirably tempered at temperatures in the range of about 400 F. to 500 F. to obtain microstructural and dimensional stability. However, tempering at these temperatures results in a substantial decrease in the hardness of the metal and accompanying reduction in load carrying ability.

It is a basic object of this invention to provide a method of heat treating bearing steels whereby a hardness in the vicinity of 63 Rockwell C is attained and the hardened metal is tempered in the range of about 400 F. to 500 F. and preferably 450* F. to 500 F. to obtain microstructural and dimensional stability without significant loss of hardness and consequent load carrying ability.

It is known that bearing steels such as SAE 52100 steel having a composition of about 0.95% to 1.10% carbon, 1.30% to 1.60% chromium, 0.25% to 0.45% manganese, 0.25 to 0.45 silicon and the balance iron with incidental quantities of sulfur and phosphorus up to about 0.025% each may be hardened by heating the steel to a temperature of about-1550 F. and quenching it in a salt bath heated to a temperature of about 5 00 F. The steel is held in the salt bath for about 1 to 1 /2 hours after which it is removed fiom the salt bath and air cooled to room temperature. This heat treatment known as austempering involves arresting the cooling at a tempera ture somewhat above the M point of the iron-carbon phase diagram and holding the metal at this temperature for a time sufficient to cause a transformation of the austenite into bainite or forms other than the highly stressed martensite. This process is outlined in the article entitled Hot Oil Quenching by D. C. Miner, FIGURE 3, page 89, April 29, 1946, issue of the Steel magazine. This process is also disclosed in the United States Patent No. 1,924,099, Bain et al.

The above process produces a steel involving a reduction in retained austenite and improved dimensional stability which are beneficial for bearing purposes. However, the hardness of this steel decreases rapidly with an accompanying reduction in load carrying ability when tempered in the range of 400 F. to 500 F. The heat treatment of the present invention produces a bearing steel which has no retained austenite, a Rockwell C hardness of 62 to 63 and a greatly improved precision elastic limit.

In general, the process of this invention involves a duplex austenitizing and quenching heat treatment immediately followed by a continuation of the quench in the form of a low temperature treatment and an extended temper. The initial austenitizing heat treatment is performed in the vicinity of about 1750 F. to 1825" F. and preferably at about 1800 F. The metal is preferably quenched in oil at about 90 F. to 100 F. Following this, the metal is reaustenitized at about 1550 F. to 1600 F. and preferably at about 1575 F. and again quenched in oil at about 90 F. to 100 F. The metal is immediately refrigerated preferably at -110 F. for about 2 hours after which it is tempered at about 450 F. to 500 F.

The initial austenitizing heat treatment is believed to increase the activity of the carbon in the carbides and induce more carbon into solution in the matrix in contrast to conventional lower temperature austenitizing heat treatments. The following quench requires only that the metal be cooled rapidly through the Ae transformation line of the iron-carbon phase diagram. The second austenitizing heat treatment and quench involves reprecipitating some of the excess highly dispersed carbon in the matrix for additional strength. This heat treatment further refines grain structure and in effect increases the carbon solution content. The low temperature treatment in effect continues the quench and involves transforming a sub stantial amount of the retained austenite into martensite without significant decreases in the hardness of the martensite. This deepfreeze treatment following the two austenitizing heat treatments and quenches makes it possible to subsequently temper the metal in the temperature range of about 450 F. to 500 F. without a decrease in hardness and loss of dimensional stability. Unless the two austenitizing and quenching treatments are employed, the deepfreeze treatment does not significantly affect the hardness of the metal, especialy when the metal is tempered in a range of 450 F. to 500 F.

Other objects and advantages of the invention will be apparent from the following detailed description of the invention.

The transformation to martensite and the complete elimination of the retained austenite from bearing steels such as the aforementioned SAE 52100 steel for maximum dimensional stability is particularly desired for instrument bearing applications.

The process of the invention is illustrated by the following detailed illustration involving a steel having stable carbides. Steel specimens were prepared of a typical SAE 52100 steel having the following chemical composition:

Three-quarter inch diameter bars were prepared which were machined into A" x /2" x test bars or specimens.

The first step of the heat treatment of this invention involves austenitizing a test bar by heating it to a temperature of preferably about 1800" F. for about 30 minutes. The duration of the heat treatment is based on the conventional austenitizing relationship of one hour heating time per inch of cross section of the metal for full conversion to austenite. This heat treatment is believed to increase the activity of the carbon in the carbides and induce more carbon to go into solution in the matrix than at lower austenitizing temperatures. After this austenitizing heating step, the metal is oil quenched preferably at a temperature of F. to F. for about 60 seconds. A suitable quench requires only that the metal be cooled rapidly through the A63 transformation line of the ironcarbon phase diagram. The resulting metallographic structure has a relatively high carbon content in the matrix, a relatively low conversion of austenite to martensite and a relatively coarse grain structure.

The test bars are now heated to an austenitizing temperature of about 1575 F. for another 30 minute period sufficient to convert the martensite into austenite. At this lower austenitizing temperature some of the excess highly dispersed carbon is reprecipitated in the matrix for additional strength. This second austenitizing heat treatment further refines the grain structure and in effect increases the carbon solution content. These test bars are also quenched at a temperature of 90 F. to 100 F. for about 60 seconds. As a consequence, a supersaturated martensite structure having a fine grain and a minute dispersion of carbon therein is developed in the metal. There remains, however, a substantial amount of retained austenite.

After the austenitizing heat treatment and quench, the metal specimens are immediately submerged for about 2 hours in a thermos bottle containing a commercial substance termed Productsol No. 340 and Dry Ice, having a temperature of about 1 F The immersion of the specimens in the refrigerated space is preferably accomplished within about 60 seconds after the second oil quench. After the 2 hour refrigeration period, the specimens are removed and permitted to warm up to room temperature. The test specimens are then tempered at progressively higher temperatures starting at 400 F. for 4 hours, than at 450 F. for 4 hours and finally at 500 F. for 2 hours. The test specimens had a Rockwell C hardness number of about 62 to 63.

Some of the test specimens which had been subjected to austenitizing treatments followed by the refrigeration treatment above described were tempered at 450 F. for

4 hours to produce metal having a Rockwell C hardness of 62.5 to 63.

Two other specimens had the Rockwell hardness of 62.5 each, a precision elastic limit of 88,000 p.s.i. each, and an elastic modulus of 29x10 p.s.i. each. Their tensile strength was 314,400 p.s.i. and 327,000 p.s.i. Standard foil strain gauge procedures were used in testing the precision elastic limit of the bars. This involved using Baldwin FA 501256 gauges applied to the test specimens by means of Eastman 910 cement in a loadunload technique to determine elastic limit. The elastic limit as used herein is defined as a load producing a first indication of permanent set in the unloaded state. In contrast, conventional instrument bearings having a Rockwell C hardness of 63 contain 2% to 3% retained austenite and a. precision elastic limit of 44,000 as determined by the same foil gauge method described above. It will be understood to those skilled in the art that the development of a 100% improvement in the precision elastic limit is of major significance for instrument bearing applications.

The process as described may be beneficially practiced with some variation in the various temperatures. The initial austenitizing may occur in the temperature range of 1750 F to 1825 F. and the second austenitizing step may be performed in the temperature range of 1550 F. to 1600 F. The quench in each instance is conveniently performed at substantially room temperature of about 90 F. to 100 F. As stated above, the quench medium must be at a sufficiently low temperature to cool the metal rapidly through the Ae transformation line. As previously indicated, the refrigeration treatment is a continuation of the second quench whereby the conversion of retained austenite to martensite is continued. This conversion process may be continued by subjecting the metal to temperatures substantially lower than the aforesaid quench temperatures. The commercial availability of equipment producing temperatures in the vicinity of l00 F. and the efiicient further conversion of the retained austenite to martensite in a period of about 2 hours at these temperatures makes the use of such cooling temperatures most efiicient.

In general, increasing the first austenitizing temperature from 1700 F. to 1800 F. increases the temper resistance of the metal with the second austenitizing temperatures of either 1475 F. or 1550 F. The hardness values are much higher than are obtained as a result of the second austenitizing treatment alone. Experiments show that quenching from a first austenitizing temperature of 1850 F. develops some coarse grained austenite effects which are present after the second austenitizing treatment. For this reason, the first austenitizing step is preferably performed at 1800 F. and temperatures in excess of 1825 F. are not employed.

The temper resistance of the metal increases with an increase in the second austenitizing temperature from 1450 F. to 15 75 F. However, when the second austenitizing temperature is raised to 1600 F., a decrease in hardness was observed in comparison with the results obtained at 1575 F. It has been found, therefore, that a second austenitizing treatment at 1575 F, coupled with an initial austenitizing treatment at 1800 F. results in optimum temper resistance of the SAE 52100 steel.

Experiments show that refrigeration of the austenitized metal, that is, cooling at temperatures in the vicinity of ll0 F., does not alfect the hardness of specimens which have been subjected to a single austenitizing treatment at 1475 F. and 1550 F. Some increase in hardness due to the refrigeration treatment was observed in specimens which were subjected to a single austenitizing F. This increase in hardness was found to persist after' tempering at temperatures as high as 500 F.

Tempering the test specimens at 500 F. tends to reduce slightly the hardness of specimens subjected to the duplex austenitizing treatment in the process of this in vention. However, the hardness of these tempered sen ples was found markedly higher than the case of single austenitized specimens even after a total of 10 hours of temper consisting of 4 hours at 400 F., 4 hours at 450 F. and 2 hours at 500 F. As indicated in the above ex amples using SAE 52l00steel, Rockwell C hardness re= sults'of 62 to 63 were obtained after this extended temper.

At the low refrigeration temperatures involved in this invention a substantial amount of the retained austefiit is transformed into martensite without significant decrease in the hardness thereof. The deepfreeze, however, trans forms only about 50% of the retained austenite. Therefore, the steel is subjected to further heat treatment to convert the balance of the retained austenite into a hard form of martensite. Following the deepfree'ze treatment, the metal is tempered preferably in the temperature range of 450 F. to 500 F. whereby substantially all of the retained austenite is converted into martensite without a significant decrease in hardness or loss of dimensional stability. The refrigeration and subsequent tempering treatments in a temperature range of 400 F. to 500 F. results in Rockwell C hardnesses of 62 to 63 in the SAE 521-00 steel. In contrast, austenitized specimens tempered which were not refrigerated had Rockwell C hardnesses of 59 to 60 even when using normal temper temperatures in the vicinity of 350 F. Those skilled in the art will understand that this involves a significant improvement for use in high stress applications such as are involved in instrumentation.

While this invention has been described by means of certain preferred embodiments and specific examples, it will be understood that the scope of the invention is not to be limited thereby except as defined in the following claims. a

I claim:

1. A process for hardening a bearing steel haw'ng a composition of about 0.95% to 1.10% carbon, 1.30% to 1.60% chromium, 0.25% to 0.45% manganese, 0.25% to 0.45 silicon and the balance iron, the steps comprising austenitizing the metal at a temperature of 1750 F. to 1825 F. and quenching the metal in a liquid quenching medium at approximately room temperature, reaustenitizing the metal at- 1550" F. to 1600 F. and quenching the metal in a liquid quenching medium at approximately room temperature, immediately refrigerating the metal at subzero temperatures to convert a major proportion of the remaining austenite to martensite and finally temperin the metal at a temperature of 400 F. to 00 F. to convert the remaining austenite to martensite.

2. A process for hardening a bearing steel having a composition of about 0.95% to 1.10% carbon, 1.30% to 1.60% chromium, 0.25% to 0.45% manganese, 0.25% to 0.45% silicon and the balance iron, the steps comprising austenitizing the metal at a temperature of 1750 F. to 1825 F. and quenching the metal in a liquid quenching medium at approximately room temperature, reaustenitizing the metal at 1550 F. to 1600 F. and quenching the metal in a liquid quenching medium at approximately room temperature, immediately subjecting the metal to Subzero temperatures for a time sufiicient to convert a major proportion of the retained austenite into maitensite and finally tempering by heating the metal at a temperature of 400 F. to 500 F. for at least 2 hours to convert substantially all of the remaining austenite to martensite.

3. A process for hardening a bearing steel having a composition of about 0.95% to 1.10% carbon, 1.30%

to 1.60% chromium, 0.25% to 0.45% manganese, 0.25% to 0.45% silicon and the balance iron, the steps comprising austenitizing the metal at a temperature of about 1800 F. and quenching the metal in a liquid quenching medium at substantially room temperature, reaustenitizing the metal at about 1575 F. and quenching the metal in a liquid quenching medium at substantially room temperature, immediately refrigerating the metal at a temperature in the vicinity of F. or less and finally tempering the metal at a temperature of 400 F. to 500 F. to convert the remaining austenite to martensite.

References Cited in the file of this patent UNITED STATES PATENTS 1,380,676 Peterson Apr. 7, 1921 2,197,365 Kjerrman Apr. 16, 1940 2,844,500 Peras July 22, 1958 OTHER REFERENCES Steel and its Heat Treatment by D. K. Bullens. Pub.: John Wiley & Sons, 1948, page 3-52 relied on.

Functions of the Alloying Elements in Steel, by E. C. Bain (presented to A.S.M.), 1939, pages 111-113 relied 

1. A PROCESS FOR HARDENING A BEARING STEEL HAVING A COMPOSITION OF ABOUT 0.95% TO 1.10% CARBON, 1.30% TO 1.60% CHROMIUM, 0.25% TO 0.45% MANGANESE, 0.25% TO 0.45% SILICON AND THE BALANCE IRON, THE STEPS COMPRISING AUSTENITIZING THE METAL AT A TEMPERATURE OF 1750*F. TO 1825*F. AND QUENCHING THE METAL IN A LIQUID QUENCHING MEDIUM AT APPROXIMATELY ROOM TEMPERATURE, REASUSTENITIZING THE METAL AT 1550*F. TO 1600*F. AND QUENCHING THE METAL IN A LIQUID QUENCHING MEDIUM AT APPROXIMATELY ROOM TEMPERATURE, IMMEDIATELY REFRIGERATIANG THE METAL AT SUBZERO TEMPERATURES TO CONVERT A MAJOR PROPORTION OF THE REMAINING AUJSTENITE TO MARTENSITE AND FINALLY TEMPERING THE METAL AT A TEMPERATURE F 400*F. TO 500*F. TO CONVERT THE REMAINING AUSTENITE TO MARTENSITE. 